The present invention relates to an overshift preventer to be mounted to the seat tube of a bicycle adjacent a front derailleur and chain wheels. Most full size bicycles sold in the United States are provided with derailleur gearing systems. These gearing systems typically consist of a front derailleur, a rear derailleur, control cables and shift levers. On a fifteen-speed or greater bike, one shift lever operates the front derailleur, permitting the chain to be forced from one of the three chain wheels to another. Rear derailleurs, provided on five speed and ten-speed bikes, shift a bicycle chain from one sprocket to another.
Problems sometimes are encountered when a chain is rapidly shifted from the largest chain wheel to the smallest chain wheel. Although rear derailleurs are provided with a spring-tensioned device to take up slack in the chain, frequently a chain may become derailed before the chain tension can be corrected. Further, the chain is more likely to derail completely when the chain is on the largest rear sprocket, because the innermost location of the large rear sprocket adjacent to the spokes tends to bias the chain inwardly. The primary cause of complete derailment is a lack of positive retention of the chain on the teeth of the sprockets, because the front derailleur guide plate is simply too far away from the sprockets to bias the chain securely onto the sprockets. When derailment occurs, the bicycle rider must coast to a stop, dismount, and re-rail the chain before continuing onward.
Front derailleurs customarily consist of a cage for guiding a chain, and a linkage mechanism for moving the cage horizontally for engaging the chain on different chain wheels. Wide-range gearing with triple chain wheels requires a particularly strong front derailleur cage. In most currently available front derailleurs the derailleur is spring biased toward the smallest, and innermost chain wheel. If the amount of travel of the front derailleur cage is not precisely adjusted using set-stop screws, and the chain either fails to engage the chain wheel properly or is shifted beyond the smallest chain wheel, then the chain becomes easily derailed.
It is an object of the present invention to provide a means for preventing the derailing of a bicycle chain adjacent a front derailleur in bicycles having wide gearing ranges (very high to very low), such as in most mountain bikes or tandem bicycles.
It is a further object of the present invention is to provide an apparatus which prevents a bicycle chain from jumping off of a larger chain wheel to a small chain wheel and failing to engage the small chain wheel.
Another object of the present invention is to provide an overshift preventer which prevents a bicycle chain from jumping off a small chain wheel due to chain bounce, which can occur in areas of rough terrain or can be caused by an object striking the drive train, or by a rapid shift.
Yet another object of the present invention is to provide a means for re-railing the derailed chain once pedalling is resumed if the chain becomes derailed from the bottom teeth of a gear, which can occur when the rider is back-pedalling or when the bike is transported on its side.
These and other objects, features and advantages of the present invention will become evident in light of the following detailed description.